The fitness industry has been in a near constant state of advancement, driven in part by the competitive desire of athletes to perform better than their opponents. Training regimens for enhancing performance have been developed and debated for several centuries. But nearly all conventional training regimens involve exercise focused on fatiguing muscles under load. Such loading exercises are often focused on improving two aspects of physical fitness: (1) endurance and (2) strength. Repeatedly fatiguing a muscle under load, also known as resistance training, according to one or more regimens is seen as a path to enhance endurance or strength, or both. For example, a long distance runner can run outdoors or on a track several hours a day in an attempt to maintain or enhance cardiovascular fitness and muscle endurance. As another example, a lineman in the National Football League (NFL) can lift heavy weights several days a week to try to maintain or enhance muscle strength.
Strength of an athlete (and endurance to some extent) is often considered a proxy for performance speed. As a result, regimens for improving performance speed, in many cases, draw from the same school of thought—that is, these regimens in many cases focus on exercising muscles under load to induce muscle fatigue in order to improve physical fitness. As an example, a running back in the NFL can train to improve sprinting speed by running with an additional load, such as additional weight or a parachute.
Regardless of whether the training is focused on improving endurance or strength there can be at least two downsides to repeatedly training muscles under load. First, repeatedly loading muscles can increase the chance of injury. For example, repeated loading can overstress a muscle or an associated joint, potentially sidelining an athlete for a recovery period. Second, regimens involving repeated loading often involve a significant time commitment. Because resistance training and training muscles under load can result in fatigue (indeed fatigue is a primary focus of such training), seriously minded exercise regimens of this type do not lend themselves to multitasking. As a result, higher level athletes typically allot a large amount of their day to focused training. Further, many such exercise regimens are performed outside to benefit from the larger available space, but are avoided during winter due to inclement weather.